Liquid crystal display devices are used in products such as watches, calculators, measuring instruments, automotive instrument panels, word processors, electronic organizers, printers, computers, televisions, clocks, and advertisement boards. Typical types of liquid crystal displays include twisted nematic (TN) displays, super-twisted nematic (STN) displays, and thin-film transistor (TFT) displays such as vertical alignment (VA) and in-plane switching (IPS) displays. These liquid crystal display devices require liquid crystal compositions that are stable to external factors such as moisture, air, heat, and light, that exhibit a liquid crystal phase over a wide temperature range centered on room temperature, and that have low viscosity and low driving voltage. These liquid crystal compositions are composed of several to tens of compounds to optimize their properties such as dielectric anisotropy (Δ∈) and refractive index anisotropy (Δn) depending on the specific liquid crystal display device.
VA displays use liquid crystal compositions of negative Δ∈, which are widely used in products such as liquid crystal display televisions. All driving modes also require low driving voltage, fast response rate, and wide operating temperature range, specifically, a large absolute value of Δ∈, a low viscosity (η), and a high nematic phase-isotropic liquid phase transition temperature (Tni). The liquid crystal compositions also require the Δn thereof to be adjusted to an appropriate range depending on the cell gap by taking into account the product of Δn and the cell gap (d), i.e., Δn×d. In addition, liquid crystal display devices used in applications such as televisions, where fast response rates are desired, require liquid crystal compositions with a low rotational viscosity (γ1).
To improve the viewing-angle characteristics of VA displays, multi-domain vertical alignment (MVA) liquid crystal display devices have been widely used. This technology divides each pixel into a plurality of domains in which liquid crystal molecules are oriented in different directions by providing protrusions on the substrate. Although MVA liquid crystal display devices have good viewing-angle characteristics, they have the following problems. The response rate of the liquid crystal molecules near the projections on the substrate differs from that of the liquid crystal molecules away from the projections. The liquid crystal molecules away from the projections have a slower response rate and thus contribute to an insufficient total response rate. The projections also decrease the transmittance. To solve these problems, polymer-sustained alignment (PSA) liquid crystal display devices (including polymer-stabilized liquid crystal display devices) have been developed. This technology, unlike normal MVA liquid crystal display devices, induces a uniform pretilt angle in each pixel domain without providing nontransparent protrusions in the cell. PSA liquid crystal display devices are manufactured by adding a small amount of reactive monomer to a liquid crystal composition, introducing the liquid crystal composition into a liquid crystal cell, and irradiating the liquid crystal composition with radiation to polymerize the reactive monomer in the liquid crystal composition while applying a voltage across electrodes. This technology allows an appropriate pretilt angle to be induced in each pixel domain and thus provides improved contrast due to improved transmittance and fast response rate due to a uniform pretilt angle (see, for example, PTL 1). Unfortunately, PSA liquid crystal display devices require a reactive monomer to be added to the liquid crystal composition. This causes many problems for active-matrix liquid crystal display devices, which require high voltage-holding ratios, and also leads to display defects such as image-sticking.
One technique has been developed to overcome the disadvantages of PSA liquid crystal display devices and to induce a uniform pretilt angle to liquid crystal molecules without mixing any substance other than liquid crystal materials in the liquid crystal composition. This technique involves mixing a reactive monomer in an alignment layer material, introducing a liquid crystal composition into a liquid crystal cell, and irradiating an alignment layer with radiation to polymerize the reactive monomer in the alignment layer while applying a voltage across electrodes (see, for example, PTLs 2, 3, and 4).
With the increasing size of liquid crystal display devices, significant changes have also been made to the process of manufacturing liquid crystal display devices.
The conventional vacuum injection process, which requires a considerable period of time for the manufacture of large-screen panels, has increasingly been replaced by one-drop filling (ODF) process (see, for example, PTL 5). This process, requiring a shorter period of time for injection than vacuum injection, has become predominant in the manufacture of large-screen panels. Unfortunately, this process presents a new problem: droplets of liquid crystal composition leave drop marks on liquid crystal display devices during manufacture. Drop marks are defined as the phenomenon where droplets of liquid crystal composition leave marks that appear white when a black image is displayed. The problem of drop marks is particularly noticeable with the above technique in which a reactive monomer is added to an alignment layer material to induce a uniform pretilt angle to liquid crystal molecules since the reactive monomer is present as a foreign substance in the alignment layer during the dispensing of droplets of liquid crystal composition onto the substrate. Although drop marks generally occur depending on the liquid crystal materials selected, the mechanism is not fully understood.
One method for reducing drop marks has been disclosed (see, for example, PTL 6). This method involves polymerizing a polymerizable compound mixed in a liquid crystal composition to form a polymer layer in the liquid crystal composition layer, thereby reducing drop marks that occur depending on the alignment control film.
Unfortunately, as in PSA technology, this method presents the problem of display image-sticking due to the reactive monomer added to the liquid crystal composition and is not sufficiently effective in reducing drop marks. Thus, there is a need to develop a liquid crystal display device with good image-sticking characteristics and few drop marks while maintaining the characteristics fundamental to liquid crystal display devices.